The present invention relates to a semiconductor laser module and, in particular, to a semiconductor laser module which comprises an external resonator having a stable oscillating condition.
In recent years, semiconductor lasers have come to be used extensively as light sources for signals and as light sources for pumping optical fiber amplifiers in the field of optical transmission. When a semiconductor laser is used as a light source for signals and as a light source for pumping an optical fiber amplifier for optical transmission, the semiconductor laser is often used in a semiconductor laser module which is a device for optically coupling laser beams from the semiconductor laser (semiconductor laser element) with an optical fiber by an optical coupling means.
In order to stabilize the wavelength of the light emitted from the semiconductor laser element, for example, this semiconductor laser module optically feed backs the light from the semiconductor laser element by a fiber Bragg grating (hereinafter, simply referred to as xe2x80x9cFBGxe2x80x9d).
However, when an external resonator comprises the FBG, conditions of deflected light fed back may have a component different from a polarization direction that can be amplified by the semiconductor laser depending on form conditions of the optical fiber such as a winding method, and the amount of light fed back effectively changes and, as a result, oscillation conditions may change.
As a countermeasure therefor, use of a birefringence fiber can be considered. By means of the birefringence fiber, a plane of polarization is maintained, the amount of light fed back can almost uniformly be maintained, and it becomes possible to suppress the fluctuation in oscillation conditions caused by the change in form such as the winding method of the optical fiber.
However, it was found that use of the birefringence fiber has a harmful influence in that peaks at same intervals arise on the spectrum and the oscillation conditions become unstable as time elapses.
Herein, where the length from the incident end of the optical coupling means side to the center of the reflective portion in the optical fiber is defined as L, the birefringence amount of the optical fiber is defined as xcex94n, and the oscillation wavelength is defined as xcex, according to the characteristics of the birefringence fiber, the propagation coefficient of light is different between eigen axes, that is, between X axis and Y axis, therefore, when the light returns after reflected by the FBG, the phase difference 4xcfx80xc2x7xcex94nxc2x7L/xcex occurs between the light propagated through the respective eigen axes.
On the other hand, the semiconductor laser element oscillates in TE (transverse electric) mode, whereas a very little amount of TM (transverse magnetic) mode also exists.
Also, when the eigen axes of the birefringence fiber are slightly shifted from the direction of TE mode of the semiconductor laser element and fixed to the module, light in is made incident onto both the X axis and Y axis in the birefringence fiber.
Also, there is no possibility that the crosstalk between the eigen axes of the birefringence fiber becomes absolutely 0.
Due to the abovedescribed factors, when light is slightly made incident into both of the X axis and Y axis and the light, which created interference due to the phase difference at propagation of each of the light, is fed back to the semiconductor element in the polarization direction that can be amplified, it is assumed that peaks at intervals of xcex2/(2xc2x7xcex94nxc2x7L) arise on the spectrum.
Such excessive peaks have been one of the causes whereby the harmful influence such that time instability occurs in the oscillation conditions of the abovedescribed semiconductor laser element with the FBG.
FIG. 6 is a schematic view of a semiconductor laser module 10 according to the prior art.
In FIG. 6, the semiconductor laser module 10 comprises a semiconductor laser element 12a and an optical coupling means 12b composed of a lens, etc. in a package 12.
Then, in the package 12, one end (incident end) of an optical fiber 13a composed of a birefringence fiber which receives light emitted from the semiconductor laser element 12a via the optical coupling means 12b is disposed and which is extended to the outside of the package 12.
An optical connector 15 is provided on the other end side of the optical fiber 13a. 14 denotes an FBG which is provided in the optical fiber 13a and comprises an external resonator.
FIG. 7 is an explanatory chart showing the relationship between the output intensity and oscillation wavelength of the semiconductor laser module 10. As shown in FIG. 7, stable oscillation with a high mode suppressing ratio cannot be performed at the center wavelength of reflection of the FBG xcexFBG, and the peaks are generated at intervals of xcex2/(2xc2x7xcex94nxc2x7L).
The present invention is made for solving such problems as the unstable oscillation conditions of the semiconductor laser module according to the prior art, and the object thereof is to provide a semiconductor laser module in which stable oscillation conditions can be provided regardless of the fluctuations in the form such as a winding method of the optical fiber.
In order to achieve the object as described above, the present invention provides a semiconductor laser module having the following construction. That is, a semiconductor laser module according to the present invention is the semiconductor laser module comprising a semiconductor laser element and an optical fiber optically coupled by an optical coupling means, wherein on the optical fiber, a reflective portion for reflecting light emitted from the semiconductor laser toward the side of the semiconductor laser element and a birefringence fiber having eigen axes and provided with birefringent characteristics are provided, said birefringence fiber is provided at least between the incident end on the optical coupling means side of the optical fiber and immediately before the reflective portion, the birefringence fiber has, at least, a connection portion where birefringence fibers are connected to each other, and at the said connection portion, eigen axes of the connected birefringence fibers are in a condition where the eigen axes are shifted in relation to each other by an established rotating angle xcex8.
The connection portion between the birefringence fibers is usually fusion-connected and preferably, where the length from the incident end on the optical coupling means to the center position of the reflective portion of the optical fiber is set as L, the connection portion between the birefringence fibers locates within the length of L/2xc2x1L/3 from the optical coupling means side.
In addition, the established rotating angle xcex8 is preferably within 90xc2x110xc2x0 or within 45xc2x110xc2x0.
The reflective portion can comprise a fiber Bragg grating, an optical connector, or a fiber cut section.
In addition, the birefringence fiber comprises an optical fiber where axially asymmetric stress is applied to the core, that is, any of a PANDA fiber whose stress applying part has a circular section, a bow-tie fiber whose stress applying part has a fan-shaped section, or an oval jacket fiber whose stress applying part has an oval section, or an oval core optical fiber where the core is oval and the waveguide structure of the core is axially asymmetric.
The optical fiber can comprise a birefringence fiber having an established length L3 provided from the center position in the longitudinal direction of the reflective portion toward the side of the optical transmission direction, and further comprise a polarization independent fiber having an established length L4 connected on the tip of the birefringence fiber having the said established length L3.
In one aspect of the present invention, the length from the incident end to the connection portion of the birefringence fiber provided between the incident end on the optical coupling means side and immediately before the reflective portion of the optical fiber is defined as L1, the length from the connection portion of the birefringence fiber to the center position of the reflective portion is defined as L2, and |L1xe2x88x92L2| is defined as xcex94L, and where xcex94Lxe2x89xa00, the established length L3 of the birefringence fiber provided from the center position of the reflective portion toward the side of optical transmission direction is 1/xcex94Lxe2x89xa61/L3.
In another aspect of the present invention, the following construction is provided. That is, the optical fiber comprises a birefringence fiber having an established length L3 provided from the center position of the reflective portion toward the side of optical transmission direction, and where the oscillation wavelength is xcex, the birefringence amount of the optical fiber is xcex94n, the length from the incident end to the connection portion of the optical fiber is L1, the length from the connection portion to the center position of the reflective portion of the optical fiber is L2, and an amount defined by the following formula containing the values of L1, and L2, and L3 as calculation values as well as containing the calculation values of all the combinations of addition and subtraction of L1, and L2, and L3 is set as Lx;
Lx=|P1L1xc2x1P2L2xc2x1P3L3|
(Pi=0 or 1, xcexa3Pixe2x89xa00, i=1, 2, 3),
the oscillation spectrum does not have any peak intervals expressed by xcex2/(2xc2x7xcex94nxc2x7Lx).
Furthermore, the present invention provides a semiconductor laser module having the following construction. That is, the semiconductor laser module comprising a semiconductor laser element and an optical fiber are optically coupled by means of an optical coupling means, wherein on the optical fiber, a reflective portion for reflecting light emitted from the semiconductor laser element toward the side of the semiconductor laser element and a birefringence fiber provided with birefringent characteristics are provided, said birefringence fiber is provided at least between the incident end on the optical coupling means side and immediately before the reflective portion, and where two orthogonal eigen axes of the birefringence fiber having different refractive indexes from each other are defined as X axis and Y axis, respectively, when the light emitted from the semiconductor laser element was made incident into the optical fiber, reflected by the reflective portion, and returned to the incident end, the value xcex94"PHgr" of the phase difference between the light made incident with its X axis polarization and returned with its X axis polarization and the light made incident with its Y axis polarization and returned with its Y axis polarization is set so as to become smaller than the value of 4xcfx80xc2x7xcex94nxc2x7L/xcex derived from the length from the incident end on the optical coupling means side to the center of the reflective portion of the optical fiber L, the birefringence amount of the optical fiber xcex94n, and the oscillation wavelength xcex.
The present invention further provides a semiconductor laser module having the following construction. That is, the semiconductor laser module comprising a semiconductor laser element and an optical fiber are optically coupled by means of an optical coupling means, wherein on the optical fiber, a reflective portion for reflecting light emitted from the semiconductor laser element toward the side of the semiconductor laser element and a birefringence fiber provided with birefringent characteristics are provided, said birefringence fiber is provided at least between the incident end on the optical coupling means side and immediately before the reflective portion, and where two orthogonal eigen axes of the birefringence fiber having different refractive indexes from each other are defined as the X axis and Y axis, respectively, when the light emitted from the semiconductor laser element was made incident into the optical fiber, reflected by the reflective portion, and returned to the incident end, the value xcex94"PHgr" of the phase difference between the light made incident by X axis polarization and returned by X axis polarization and the light made incident by X axis polarization and returned by X axis, polarization is greater than the value of 4xcfx80xc2x7nLDxc2x7LLD/xcex derived from the refractive index of the laser diode (chip) nLD, the resonator length of the semiconductor laser (element) LLD, and the oscillation wavelength xcex.
According to the semiconductor laser module having the abovedescribed construction, for stabilizing the oscillation conditions, the phase difference between the eigen axes of the birefringence fiber (hereinafter, simply referred to as xe2x80x9cphase differencexe2x80x9d) at the time when the light was made incident and returned is manipulated, thereby the object is achieved.
In the present invention, the phase difference is manipulated by two concrete manipulations, that is, by rotating the eigen axes of the birefringence fiber at the midway (connection portion) of the external resonator by an established angle xcex8 and by appropriately adjusting the position of the connection portion in the longitudinal direction of the optical fiber, and as a result, the oscillation conditions are stabilized.
By rotating and connecting the axes of the birefringence fiber, only the optical phase is manipulated and the original effect to maintain polarization is not lost, thereby the amount of light fed back is uniformly maintained.
However, if a phase difference exists there, peaks arise at intervals of xcex2/(2xc2x7xcex94xc2x7Lx) on the oscillation spectrum. Herein, xcex is a oscillation wavelength of the external resonator, xcex94n is a refractive index difference between the eigen axes of the birefringence fiber (hereinafter, referred to as xe2x80x9cbirefringence amountxe2x80x9d.
Therefore, in the present invention, by decreasing the phase difference, such peak intervals are made very wide, for example, wider than the gain spectrum of the semiconductor laser element or, when an FBG is used, the peak intervals are made wider than its reflection spectral bandwidth, thereby eliminating or significantly diminishing peaks that arise on the oscillation spectrum.
Preferably, when the phase difference can be eliminated, peak intervals become infinite and, in essence, peaks do not exist, therefore, the oscillation conditions of the external resonator are stabilized.
On the other hand, by greatly increasing the phase difference, the peak intervals are made narrower than the longitudinal mode intervals calculated based on the oscillator length LLD of the semiconductor laser element, thereby reducing influence due to the phase difference on the oscillation conditions.
Herein, by connecting the axes of the birefringence fiber after rotating them at 90xc2x0 to each other at the midpoint of the birefringence fiber composing a part of the external resonator, the phase difference is nearly eliminated.
However, in such method, since a connection portion is formed on the optical fiber, it is preferable to take reflection at the connection portion into consideration. It is because it is actually impossible to make the connection portion without producing reflection and, as far as the connection has any reflections, there is a possibility that even very small reflection can influence phases between the eigen axes.
Also, when an FBG is used as a reflective portion, since complex amplitude reflectance accompanied by phase changes must be taken into consideration, its phase prerequisites are also different from a simple reflection plane.
Namely when the connection portion is provided at the midpoint of the birefringence fiber composing the external resonator, because of the reasons described above, the phase difference cannot actually become 0.
In such a case, where the length of the birefringence fiber is defined as L, the length from the incident end to the connection portion of the birefringence fiber is defined as L1, and the length from the connection portion to the center position of the reflective portion of said birefringence fiber is defined as L2, it becomes possible to manipulate phase conditions of the light which returns to the semiconductor laser element not by changing the prerequisites of L=L1+L2, L1=L2=L/2 but by appropriately changing the size of L1 and L2.
In addition, in a case where the reflection from the optical fiber end face on the optical transmission direction with respect to the FBG is taken into consideration, it is necessary to optimize not only L1 and L2 but also the relationship between xcex94L=|L1xe2x88x92L2| and L3. When xcex94Lxe2x89xa00, the influence is small if peak intervals xcex2/(2xc2x7xcex94nxc2x7L3) made to occur by L3 are wider than intervals xcex2/(2xc2x7xcex94nxc2x7xcex94L) based on xcex94L. That is, selecting L3 so as be 1/xcex94Lxe2x89xa61/L3 is appropriate.
As described above, in the present invention, by appropriately selecting L1, L2, and L3, peaks of all wavelength intervals calculated based on Lx are more optimally suppressed, thereby enabling the external resonator to oscillate stably.
Furthermore, a semiconductor laser module having the birefringence fiber without an FBG is also important in practical use. Since the semiconductor laser module is very sensitive to reflection from various spots, if it has no FBG, a connector and a cut section of an optical fiber can be regarded as external feed back portions without wavelength selectivity. The cut section of the optical fiber may be cut vertically or diagonally with respect to the longitudinal direction of the optical fiber.
According to the present invention, even in a situation where reflection from the connector or cut section of the optical fiber has an influence, by composing the appropriate external resonator as described above, stable oscillation can be realized.
Also, in a case where the eigen axes of the birefringence fiber are rotated at 45xc2x0 and are fusion-connected to each other, although phases cannot completely be compensated, there is an effect that reduces interactions between the eigen axes.
Furthermore, according to the present invention, a polarization independent fiber can be connected on the tip of the birefringence fiber. In such case, there is an advantage such that loss due to the difference in mode fields, etc. is small when another polarization independent fiber is connected by means of a fixture.
As has been described above, according to the semiconductor laser module of the present invention, by using a birefringence fiber of the optical fiber composing an external resonator for the semiconductor laser module which has an unstable oscillation condition based on fluctuations in the form such as the winding method of the optical fiber, a semiconductor laser module which stably oscillates can be achieved while the oscillation mode of the semiconductor laser module having the external resonator maintains a high mode suppressing ratio.